U.S. Pat. No. 6,790,386 describes a dielectric fluid comprising an iso-paraffin base oil and additives. The iso-paraffin base oil is prepared by hydrotreating, hydroisomerisation and hydrogenation of a paraffinic vacuum feedstock.
U.S. Pat. No. 5,912,212 describes oxidative stable oil lubricating formulations consisting of a hydrocracked paraffinic mineral base oil, 3-methyl-5-yert-butyl-4-hydroxy propionic acid ester, dioctylaminomethyltolyl-triazole and dilaurylthiodipropionate. The oil had a high oxidative stability.
WO-A-02070629 describes a process to make iso-paraffinic base oils from a wax as made in a Fischer-Tropsch process. According to this publication base oils having a kinematic viscosity at 100° C. of between 2 and 9 cSt can be used as base oil in formulations such as electrical oils or transformer oils.
There is a desire to formulate electrical oils using a base oil having the properties of the Fischer-Tropsch derived base oil as described in WO-A-02070629. The main reasons are the excellent low-temperature properties of said base oils in combination with the relatively simple process to make said base oils as compared to similar oils prepared from mineral crude sources.
Electrical oil formulations require certain properties in order to be applicable for use. Typical requirements are that sludge formation should be low, oxidation stability should be high, cold flow properties should be adequate for its intended use, flash point should be adequate for its intended use and the dielectric dissipation factor should remain low, even after prolonged testing at elevated temperature. In particular for applications that require high performance at elevated temperatures and wherein elevated peak temperature in the electrical oil formulation are occurring, a very high flash point is required. At the same time, the formulation should still have good low temperature performance.
Applicants further found that formulating an electrical oil formulation starting from such a synthetic iso-paraffin base oil is not straightforward as compared to when starting from a mineral based paraffinic base oils. The object of the present invention is to provide an electrical oil formulation, which has adequate properties for its use. This object is achieved in the following oil formulation.